My opinion is just based on a reading of their 17-page filing today. I totally agree that they aren't an 'real-interest' party by the normal definition. However, by the way Congress wrote the rule, I think RPX has reasonable case that they should still get to contest the patent.
I think that Congress's rule may be unconstitutional and may be (probably should be) invalidated by SCOTUS, but the question is not whether the RPX, it's simply whether there is an inconsistency with Congress's rule and Ariticle 3, and it appears that there is.
Don't get me wrong, I hope SCOTUS says no. Furthermore, RPX and defendants should ultimately lose simply based on the validity of the patents either way... I just think that after reading today's filing, it's a reasonable argument. I'd be curious to hear what your thoughts are on their filing.